Numerous designs have been proposed for recovering the fuel vapors displaced from a vehicle fuel tank as it is filled. Typically, a valve near the top of the tank filler pipe, known as a tank vent valve assembly or TVVA, selectively opens and closes a vapor line running from the top of the filler pipe to a vapor storage canister. The TVVA is open during the filling of the tank, to allow fuel fill vapors to go the the canister, but closed when the filler pipe is closed, so as to prevent liquid fuel from splashing up the filler pipe and into the vapor line. Many designs also provide further protection for the canister in the form of a buoyant float that rises to engage an open seal seat and block the vapor vent line in the event of liquid fuel overfill rising up to the end of the filler pipe at the end of the fuel fill.
A typical example of such an overfill protection float may be seen in U.S. Pat. No. 4,747,508 to sherwood, assigned to the assignee of the subject invention. Such a float should be very light in order to be sufficiently buoyant to shut off quickly in the event of overfill. However, the fuel fill vapors need a free exit path to the canister in order to prevent vapor pressure build up in the tank. Furthermore, the fuel fill vapors, which rush past the float and through the seal seat into the vapor vent line, may be traveling rapidly and have a significant volume, particularly in the case of a large fuel tank that is being rapidly filled. In a phenomenon known as "corking," the force of the rushing fuel vapors may physically lift the float up against the seat, thereby blocking free vapor exit to the canister during fill.
The float corking problem has been recognized in at least one instance. U.S. Pat. No. 4,714,172 to Morris shows a chamber interposed between the vapor vent line and the filler pipe that contains a round, buoyant float. The float chamber does not confine the float closely. Instead, the chamber has a spherical inner surface that is larger than the float, and the seal seat opening is located at the top of the sphere. The fuel fill vapors are apparently directed into the chamber so as to hit the float off center cause it to spin. The spinning float is supposed to continue to spin around the spherical inner surface of the chamber faster and faster, centrifugal force thereby keeping it away from the open seal seat. It would appear that the vehicle would have to be sitting in an attitude during fuel fill which assured that the seal seat was at top dead center of the float chamber, and that any shaking of the vehicle filler pipe by the filler nozzle might disturb the ball from its spinning orbit and into the seal seat.